Quasars: Black holes and bright lights

25 January 2016

Interview with

Dr Manda Banerji, Cambridge University Institute of Astronomy

Supermassive black holes, which sit at the centres of galaxies, can have massesQuasar ten billion times greater than the Sun,. And despite being black holes capable of locking in light, they've actually been linked to some of the brightest lights in the Universe: quasars. Manda Banerji, who is from the Institute of Astronomy at Cambridge University, tells Chris Smith about these shining lights... 

Manda - Quasars,  these are the brightest lights that we see in the universe and, basically, what they are are powerful, very energetic emissions but they're coming from the immediate vicinity of these supermassive black holes.

Chris - Are they visible light that I could see with my own eyes, or would I need a fancy telescope to see this sort of light?

Manda - Quasars do emit in visible light, but they also emit in all parts of the electromagnetic spectrum, so all the way from x rays to radio waves.

Chris - And is that the clue that this is not just a star we're dealing with here because the fact that you're seeing all these different ranges of colours of light - all these different wavelengths - and stars don't do that?

Manda - Yes, absolutely.  So that is one of the biggest clues; all of this emission and all parts of the electromagnetic spectrum that we're dealing with something very powerful, very energetic.

Chris - And what do you think's going on then to do this?

Manda - So what you need to do to power a quasar is to basically dump a whole load of material, a whole load of matter, onto these supermassive black holes at the centres of galaxies.  So, when you have lots of material, lots of gas, dust, stars being funneled into these central regions, all of that material gets heated up and it shines very brightly as a quasar.

Chris - So this is sort of like a black hole with indigestion.  It's lots of material trying to pile in very, very fast?

Manda - Yes.  It's a feeding monster of a black hole, so quasars are really the feeding phase of a black hole.  We're seeing them as they are devouring all the material up around them.

Chris - Do they have any effect on the galaxy on which they sit, by virtue of just existing?

Manda - Yes.  So this is one of the main reasons that we study quasars is because we now know that they have a very profound impact on their host galaxies.  So we now know that there's a supermassive black hole living at the centres of most galaxies in our universe and when these supermassive black holes feed during this quasar stage, all of this energy that they're outputting into their host galaxy, that can have a very dramatic impact that can blow gas, dust, stars out of the whole galaxy and so in that way it can actually control how big a galaxy will eventually get.

Chris - So it does have an important modelling effect on the ultimate structure of the galaxy.  Can you also look at it the other way, which is to say, if I look at the structure of the quasar and its behaviour, can I deduce something about the black hole that is giving rise to it by studying it, and that tell me something about what the black hole is doing itself?

Manda - Yes, absolutely.  So, we look at this quasar emission and by looking at the gas and dust that's being eaten up by this black hole, we can infer lots of interesting things about the black hole like, how big it is, how massive it's going to get.

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